(Field of the Invention)
The present invention relates to a cathode active material for a lithium ion battery, a cathode material for a lithium ion battery, and a lithium ion battery.
(Description of Related Art)
Lithium ion batteries have been widely used as small-sized power sources for mobile information terminals and the like because of their high energy density. In recent years, lithium ion batteries have also been utilized as large-sized power sources for electric vehicles, hybrid-EVs, or power storage.
In some applications as large-sized power sources, a number of lithium ion batteries are used in multiple series. To achieve a battery with a higher energy density or to decrease the number of batteries connected in series, a lithium ion battery is required to have a higher voltage than a conventional lithium ion battery. For this reason, the lithium ion battery needs a cathode active material that stably exhibits a high potential of 4.5 V or more based on lithium metal as a reference.
As such a cathode active material, a spinel-type composite oxide is known in which a specific transition metal substitutes for a part of Mn in a compound represented by a general formula LiMn2-xMxO4 (where M is Ni, Co, Cr, Fe, Cu, etc.).
In particular, in the spinel-type composite oxide with Ni as M (hereinafter referred to as a “5 V spinel”), a change in valence of Ni stabilizes and exhibits the high potential of around 4.7 V. The high potential capacity is substantially proportional to the amount of substitution x in the above-mentioned general formula. The theoretical composition LiMn1.5Ni0.5O4 is known to have, in theory, a discharge potential of 4.7 V as a whole.
Japanese Patent Application Laid-Open No. 2002-75366 (Patent Document 1) discloses a cathode active material that is a manganese-based spinel-type composite oxide containing fluorine as a constituent element in terms of improving charge-discharge cycle characteristics, particularly, cycle characteristics under a high-temperature atmosphere.
Japanese Translation of PCT International Application Publication No. JP-T-2008-536285 (Patent Document 2) discloses a cathode active material having a cubic structure and represented by a spinel Li1+a[Ni0.5Mn1.5-xMx]O4-bFb (where M is at least one metal selected from the group consisting of Co, Ni, Cr, Mg, Al, Zn, Mo and W), with a fluorine compound covering the surface of the cathode active material.
Japanese Patent Application Laid-Open. No. 2003-229130 (Patent Document 3) discloses a cathode active material having a high capacity and a high voltage of 4.5 V or more for Li. The cathode active material is formed by substituting any one of the monovalent to trivalent metal elements lighter than Mn for a part of Mn and also by substituting F or Cl for O in Li(NizMn2-x)O4 (0.4≦x≦0.6).